can: m_can: Count read getindex in the driver

The getindex gets increased by one every time. We can calculate the
correct getindex in the driver and avoid the additional reads of rxfs.
Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Link: https://lore.kernel.org/all/20221206115728.1056014-6-msp@baylibre.comSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/m_can.c

@@ -477,19 +477,16 @@ static void m_can_receive_skb(struct m_can_classdev *cdev,
 	}
 }
 
-static int m_can_read_fifo(struct net_device *dev, u32 rxfs)
+static int m_can_read_fifo(struct net_device *dev, u32 fgi)
 {
 	struct net_device_stats *stats = &dev->stats;
 	struct m_can_classdev *cdev = netdev_priv(dev);
 	struct canfd_frame *cf;
 	struct sk_buff *skb;
 	struct id_and_dlc fifo_header;
-	u32 fgi;
 	u32 timestamp = 0;
 	int err;
 
-	/* calculate the fifo get index for where to read data */
-	fgi = FIELD_GET(RXFS_FGI_MASK, rxfs);
 	err = m_can_fifo_read(cdev, fgi, M_CAN_FIFO_ID, &fifo_header, 2);
 	if (err)
 		goto out_fail;
@@ -554,6 +551,9 @@ static int m_can_do_rx_poll(struct net_device *dev, int quota)
 	struct m_can_classdev *cdev = netdev_priv(dev);
 	u32 pkts = 0;
 	u32 rxfs;
+	u32 rx_count;
+	u32 fgi;
+	int i;
 	int err;
 
 	rxfs = m_can_read(cdev, M_CAN_RXF0S);
@@ -562,14 +562,17 @@ static int m_can_do_rx_poll(struct net_device *dev, int quota)
 		return 0;
 	}
 
-	while ((rxfs & RXFS_FFL_MASK) && (quota > 0)) {
-		err = m_can_read_fifo(dev, rxfs);
+	rx_count = FIELD_GET(RXFS_FFL_MASK, rxfs);
+	fgi = FIELD_GET(RXFS_FGI_MASK, rxfs);
+
+	for (i = 0; i < rx_count && quota > 0; ++i) {
+		err = m_can_read_fifo(dev, fgi);
 		if (err)
 			return err;
 
 		quota--;
 		pkts++;
-		rxfs = m_can_read(cdev, M_CAN_RXF0S);
+		fgi = (++fgi >= cdev->mcfg[MRAM_RXF0].num ? 0 : fgi);
 	}
 
 	return pkts;